Every year in the United States, more than two thousand children are born with an intersex condition or disorder of sex development. What makes someone a boy or a girl? Is it external genitalia, chromosomes, DNA, environment, or some combination of these factors? Not even doctors or scientists are entirely clear. What is clear is that sex is not an either-or proposition: not girl/boy, XX/XY, switching between two poles like an on-off switch on a radio. Rather, sex is like the bass and treble knobs on that radio.Between XX and XY provides a fascinating look at the science of sex and what makes people male or female. There are people born XXY, XXXY, or XXXXY, or with any number of variations in X or Y chromosomes, but those who do not fit into society's preconceived notions about sex often face a difficult path in life.

Dr. Callahan explores why humans are so attached to the idea of two sexes, and examines our obsession with sex and sexual intercourse through the ages.